This invention relates to a position measuring system including a node which has a radio transmitter and an access point used as a radio base station in the system.
As a representative conventional node position measuring method, there has been available a method of measuring a position by using a signal from a satellite such as a GPS.
As another node position measuring method, there has been available a method of receiving signals sent from a plurality of base stations by a node and calculating a position of the node based on a reception timing difference of the signals.
Specifically, regarding a cellular telephone system, there has been proposed a method of calculating reception timing differences of signals sent from base stations to a mobile terminal (propagation time differences of signals T1−T2 and T3−T2 from the base stations to the mobile terminal), multiplying the propagation time differences by a light velocity to calculate propagation distance differences of signals from the mobile terminal to the base stations by equation (1).D1−D2=c(T1−T2)D3−D2=c(T3−T2)  (1)
And a position of the moving terminal is detected (refer to JP7-181242A).
Furthermore, regarding a radio LAN system, there has been proposed a method of calculating a reception timing difference of signals sent from a node (terminal) and received by base stations (reception timing difference Ti−T1 between the base stations), multiplying the reception timing difference by a light velocity to calculate a propagation distance difference of the signals from the node to the base stations by equation (2).{|P−Pi|−|P−P1 |}=c(Ti−T1), i=2, . . . , n  (2)
And a position of the node is detected (refer to Atsushi Ogino et al. p. 662 B-5-203, “Wireless LAN Integrated Access System (1) Studies on Position Detecting System”, 2003 General Meeting Collected Lecture Papers, Institute of Electronics, Information and Communication Engineers).
In the method in which the signal from the satellite such as a GPS is used, a special receiver or antenna is necessary, making it difficult to miniaturize the node and to achieve low power consumption. This method can only be used outdoors to receive a radio wave from the satellite. Thus, if the node is used indoors, there is a method of disposing receiving stations at several places to receive radio waves from the node and approximating a position of a receiving station which can receive a signal to that of the node. According to this method, however, since position measuring accuracy depends on a distribution density of the receiving stations, many receiving stations must be distributed to detect an accurate position of the node.
Additionally, in the method of causing the node to receive the signals sent from the plurality of base stations and: calculating the position of the node based on the reception timing difference, clocks of the base stations must be synchronized with one another before the sending of the signals. Accordingly, it is impossible to measure a position at the moment when the node detects an abnormality. Besides, the node needs to have a receiver, so that it makes difficult to miniaturize the node and to achieve lower power consumption.